The invention pertains to laser light sources and particularly to vertical cavity surface emitting lasers. More particularly, the invention pertains to long wavelength lasers.
A vertical cavity surface emitting laser (VCSEL) may include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL may be driven by a current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack. The first contact may instead be on top of the first mirror stack in a coplanar arrangement.
VCSEL mirror stacks are generally formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlAs/AlGaAs material system wherein the different refractive index of each layer of a pair may be, for example, achieved by altering the aluminum content in the layers. In some devices, the number of mirror pairs per stack may range from 20 to 60 to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of pairs may increase the percentage of reflected light.
In many VCSELs, conventional material systems may perform adequately. However, new products are being developed requiring VCSELs to emit light having long wavelengths. VCSELs emitting light having a long wavelength are of great interest in the optical telecommunications industry because of the low fiber dispersion at 1310 nanometers (nm) and the low fiber loss at 1550 nm. For instance, a long wavelength VCSEL may be obtained by using a structure having an InGaAs/InGaAsP (or InAlGaAs) active region. When an InGaAs/InGaAsP active region is used, an InP/InGaAsP (or InAlGaAs /InAlAs or InAlGaAs/InP) material system needs to be used for the mirror stacks in order to achieve a lattice match relative to the InP substrate. The lattice matching between the substrate and the layers should be substantially close to ensure a true single crystal film or layer growth.
In the InP material based system, it is difficult to achieve a suitable monolithic DBR -based mirror structure having a reasonable thickness because of the insignificant difference in the refractive indices in this material system. As a result, many layers, or mirror pairs, are needed in order to achieve useful reflectivity. Useful reflectivity may be 99.8 percent or greater. Numerous attempts have been made to address the problem of very thick mirror structures. One attempt included a wafer bonding technique in which a DBR mirror is grown on a separate substrate and bonded to the active region. This technique has had only limited success and also the interface defects density in the wafer fusion procedure causes potential reliability problems. Other approaches to making satisfactory long wavelength VCSELs have been fraught with one problem or another. For instance, lattice matched InP based mirrors used for 1550 nm VCSELs have a host of problems in growth, processing, and optical performance. The low index contrast of InGaAsP (or InAlGaAs) and InP (or InAlAs) leads to the requirement of extremely thick (ten microns or thicker) DBRs of 45 or more mirror periods or layer pairs. The AlGaAsSb or AlGaPSb systems associated with an InP substrate may be difficult to grow by MOCVD, and with good contrast, may still require at least 25 mirror pairs to achieve adequate reflectivity for VCSEL operation. For some VCSEL structures, such as those having a long wavelength, current confinement is an important characteristic. Proton implantation and lateral oxidation have been developed and used for current confinement in vertical cavity surface emitting lasers (VCSELs), especially GaAs-based VCSELs. For some VCSELs, however, proton implantation and lateral oxidation cannot be easily applicable due to either very thick top DBR stacks for proton implantation or lack of lattice-matched high aluminum containing material for oxidation, respectively. This is particularly the case of InP related materials for long wavelength VCSEL operation. For InP based material systems, since index contrasts are relatively small as compared to GaAs based counterparts, the DBR stacks tend to be much thicker to provide reasonable reflectivity. Consequently, large amounts of energy are required for gain guide proton implantation, which is not practical. Such energy levels may damage other parts of the structure. Also, the aluminum content is significantly lower in materials lattice matched to InP substrates than in those materials lattice matched to GaAs substrates. The low aluminum content makes lateral oxidation difficult. Thus, an alternative to implantation and oxidation for making a gain guide is needed. The invention provides that alternative.